(1) Field of the Invention
The present invention relates generally to ion implanters, and more particularly to an ion source having new shields for improved cleaning and maintenance of the ion source.
(2) Description of the Related Art
In semiconductor manufacturing, ion implantation is primarily used to introduce dopant ions into silicon wafers. This is accomplished by generating, in an ion implanter, a gas plasma such that the resultant particles can be accelerated under the influence of electric field, and directed onto a semiconductor substrate for implantation to a desired depth beneath the surface of the substrate. Because of its superiority over chemical doping, ion implantation has replaced diffusion (chemical) doping in an increasing number of VLSI (very large scale integration) applications. Along with it, however, it has brought some new problems--cleaning and maintenance of the equipment being one of the important ones--both from an operational point of view, as well as the safety of workers.
In general, problems associated with cleaning and maintenance are exacerbated by the complexity of the ion implanters used in the fabrication of VLSI. Complexity arises from the many subsystems that the implanters usually have. Each of the subsystems contain several components and they are exposed to materials which can be extremely hazardous. Most commonly used materials, called feed source, for implanting silicon are boron (B), phosphorous (P) and arsenic (As). They can be lethal when supplied in gaseous form, such as BF.sub.3, PH.sub.3, AsH.sub.3, or less toxic when generated from solid sources. The insides of chambers that are exposed to these materials must be cleaned at regular intervals as a preventive maintenance measure.
One of the subsystems that is critical to the operation of ion implanters is a chamber called the ion source where feed source material is ionized. Ionization is accomplished usually by first heating the feed source molecules to a desired temperature and vaporizing it. Then the particles are directed to an arc chamber where ions are formed typically through collision with electrons from an arc discharge. An ion extraction and analyzing device next selects only the ion species of interest, and rejects all others forming an ion beam that is separated from the remaining species by an analyzing magnet. By adjusting the magnetic filed strength, only the ionic specie of interest is given a particular trajectory that allows it to pass through a resolving slit (aperture) and into an acceleration tube. The evacuated tube creates the acceleration field to adjust the ion energy to the desired energy level, focuses the ion beam to a particular size and shape, and distributes the ions uniformly over the target area, namely, the semiconductor substrate. Of the subassemblies that is most exposed to many different specie of the hazardous feed source is the ion source. This is seen in FIG. 1.
In the ion implantation system shown in FIG. 1, ion generating source (10) provides the needed ion beam (30) which is formed by an ion extraction electrode assembly (20). The path of ion beam (30) is controlled by analyzing magnet (45) and the beam is directed into acceleration tube (70) and then on to implantation chamber (50). It should be noted that analyzing magnet (45) is supported by a high-voltage system which is not shown in FIG. 1; neither is a movable system that allows the implantation chamber (50) to be aligned relative to the ion beam (30) for they do not directly pertain to the aspects of this invention. Suffice it to say that ion beam (30) impinges upon one or more silicon wafers that are automatically placed on a rotating platform (55) by means of a robot arm (60) that transfers wafers back and forth between a wafer holder (62) and the platform (55) through a vacuum port (64). Ion beam (30) impacts each of the wafer and selectively dopes those wafers with ion impurities. It will be observed that subassemblies other than the ion source (10) are exposed to only the ions of selected beam (30).
The ion implanter shown in FIG. 1 is of the high current type with maximum beam current of about 10 milliamps (mA) and energies ranging between about 10 to 160 kilo electron volts (keV), and includes a spinning platform for moving multiple silicon wafers through the ion beam. Ion beam (30) is wide enough to impact an entire wafer surface as the platform rotates each wafer though the ion beam. Another type of implanter, known as medium current implanter, treats one wafer at a time with a total beam current of up to 2 mA and maximum energies of about 200 keV. The medium current implanters use beam shaping electronics to deflect a relatively narrow beam from its initial trajectory to selectively dope or treat the entire wafer surface. In comparison with the medium current implanters, it is found that the particulate residues and contaminants left behind in the ion source chamber of high current implanters are much denser and cover much larger surface area. Also, high current implanters contain more components in their ion source chamber and therefore are more susceptible to collecting particulates during the process of ionization.
One of the additional components that have a large area and that is usually covered with particulate matter generated during ionization is the cold plate. It will be known to those skilled in the art that cold-plate is necessitated because of the high temperatures encountered with high current implanters. Furthermore, the cold-plate is usually not easily accessible through the access door to the ion source.
Generally, dismounting parts of an ion implanter for purposes of cleaning is very time consuming and labor intensive. On the other hand, reaching into the interior of the ion source, cleaning the chamber walls, other surfaces around the cold-plate as well as around extraction electrode assembly is difficult and hazardous for personnel responsible for preventive maintenance. In prior art, there have been attempts made--and successfully--at shielding certain components, but only locally rather than globally for the entire interior of an ion source. In U.S. Pat. No. 5,497,006, for example, heated filament cathode is shielded from plasma stream locally so that the life of the filament can be extended. No shielding attempt is made in U.S. Pat. No. 4,719,355 even when dopants are vaporized from solid in a crucible inside the ion source. The interior walls, and other components inside the ion source are covered with particulate matter emanating from the crucible. It is therefore the purpose of this invention to disclose techniques for facilitating the protection of the largest portion of the interior surfaces of an ion source and other components that reside inside the ion source. It is believed that the disclosures herein will result in more cost effective maintenance of ion implanters with much reduced hazard to workers on the semiconductor manufacturing line than presently available in the state of the art.